This invention relates to a process for producing isopulegol which is a useful material for making aromas and an important precursor for synthesizing menthol.
Menthol, particularly l-menthol is extraordinary important as an aroma with coolness and has been finding wide applications in a variety of fields. Known methods of synthesizing menthol are divided into optical resolution of dl-menthol and asymmetric synthesis (see Indoh Moto-ichi, Gosei Koryo, Kagaku Kogyo Nipposha, pp. 106-114). In the route of asymmetric synthesis, l-menthol is obtained by hydrogenation of l-isopulegol as a precursor. Synthesis of the l-isopulegol involves selective cyclization of d-citronellal as an important step.
The process of producing l-isopulegol comprising selectively cyclizing d-citronellal in the presence of zinc bromide as a catalyst, which is disclosed in JP-A-53-116348 and Nakatani and Kawashima, Synthesis, p. 147 (1978), has already been put to practical use. In this process, the ratio of l-isopulegol to other isomers is 94/6.
Other catalysts hitherto reported for selective cyclization of citronellal include Me2AlCl (Micheal, K. and Snider, B. B., J. Am. Soc., 102, pp. 7951-7953 (1980)), Zn(binaphthol) (Sakane, S. et al., Tetrahedron Lett., vol. 26, No. 45, pp. 5535-5538 (1985)), RhCl(PPh)3 (Funakoshi, K. et al., Chem. Pham. Bull., vol. 37, No. 8, pp. 1990-1994 (1989)), an acid catalyst supported on a silica gel or alumina carrier (Kropp, P. J., J. Org. Chem., vol. 60, pp. 4146-4152 (1995)), Zn/trimethylsilyl chloride (Marty, M. et al., Tetrahedron, vol. 52, No. 13, pp. 4645-4658 (1996)), SmI2 (Sarkar, T. K. and Nandy, S.K., Tetrahedron Lett., vol. 37, No. 29, pp. 5195-5198 (1996)), Sc(trifluoromethane sulfonate)3 (Aggarwal, V.K. et al., Tetrahedron Lett., vol. 39, pp. 1997-2000 (1998) and International Publication WO 99/32422), trans-[Ru(salen) (NO) (H2O)]+ (Ellis, W. W. et al., Chem. Commun., pp. 1311-1312 (1998)), S-ZrO2 (Yadav, G.D. and Nair, J.J., Chem. Commun., pp. 2369-2370 (1998)), and solid catalysts (JP-A-11-267524). None of these catalysts is superior in selectivity to isopulegol to zinc bromide that have been used in industry.
Sc(trifluoromethane sulfonate)3 is equal to zinc bromide in selectivity only when the reaction is conducted in an extremely low temperature of xe2x88x9278xc2x0 C. The selectivity to l-isopulegol reduces to 80% at room temperature, which is insufficient for industrialization.
The catalyst of the present invention, a tris(2,6-diarylphenoxy)aluminum, is reported as a catalyst for polymerization (JP-A-11-335432) or aldol condensation (Yamamoto, H. et al., J. Am. Chem. Soc., vol. 116, pp. 4131-4132 (1994), Yamamoto, H. et al., J. Am. Chem. Soc., vol. 122, pp. 7847-7848 (2000)). Application of this catalyst to cyclization reactions, still less to synthesis of isopulegol, has not been reported nor known.
An object of the present invention is to provide a process for obtaining isopulegol through high-selectivity cyclization of citronellal, particularly a process for producing l-isopulegol, which is an important synthesis precursor for l-menthol and is useful as a material of perfumes, etc., by simple operations with safety in high yield.
As a result of extensive investigations, the present inventors have found that a specific catalyst enables citronellal to be cyclized to provide isopulegol in high yield and at such a selectivity as high as 98% or more based on the total isomers produced, i.e., isopulegol, isoisopulegol, neoisopulegol, and neoisoisopulegol. The present invention has been completed based on this finding.
The present invention includes the following embodiments.
1) A process for producing isopulegol represented by the following formula (1): 
which comprises selectively cyclizing citronellal represented by the following formula (2): 
in the presence of a tris(2,6-diarylphenoxy)aluminum catalyst represented by the following formula (3): 
wherein Al represents an aluminum atom, Ar1 and Ar2 each represent a substituted or unsubstituted aryl group or a heteroaryl group, and R1, R2 and R3 each represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atom(s), an alkoxy group having 1 to 8 carbon atom(s), a substituted or unsubstituted aryl group, a dialkylamino group wherein each alkyl group has 1 to 4 carbon atom(s), or a nitro group.
2) A process for producing optically active isopulegol represented by the following formula (4): 
wherein * indicates an asymmetric carbon atom, which comprises selectively cyclizing optically active citronellal represented by the following formula (5): 
wherein * indicates an asymmetric carbon atom, in the presence of a tris(2,6-diarylphenoxy)aluminum catalyst represented by the following general formula (3): 
wherein Al, Ar1, Ar2, R1, R2, and R3 are as defined above.
3) The process described in (1) or (2) above, wherein the tris(2,6-diarylphenoxy)aluminum catalyst is a reaction product obtained by reacting at least one aluminum compound selected from an alkylaluminum compound represented by the following general formula (6a):
(R4)3-pAlHpxe2x80x83xe2x80x83(6a)
wherein Al represents an aluminum atom, R4 represents an alkyl group having 1 to 4 carbon atom(s), and p represents an integer of 0 to 2, and a metal aluminum hydride represented by the following general formula (6b):
MAlH4xe2x80x83xe2x80x83(6b)
wherein M represents a lithium atom, a sodium atom or a potassium atom, and Al represents an aluminum atom; and a 2,6-diarylphenol represented by the following general formula (7): 
wherein Ar1, Ar2, R1, R2, and R3 are as defined above, in an inert solvent.